In consumer electronic products, the thickness and or volume of the product is a very important sales factor. The thickness is termed “Z-stackup”. Flexible printed circuit boards are the material of choice for component mounting and interconnect due to low cost and thin Z-stackup. In some cases a flexible printed circuit board requires a rigid area for mounting certain components to the flexible printed circuit board. Some products, for example cell phones, need a low ohmic resistance (a maximum of two ohms for some customers) means for the common grounding of electrically conducting components and subsystems. Ideally the grounding means should provide for the design of a Faraday cage around all except the radiating portion of the antenna. For example, in a cell phone, a camera module must provide for its external grounding to the phone chassis or other components. Some applications require the provision of an electrically active ground pad to the bottom side of the imager at the flexible printed circuit board level. This to be done with little or no increase in Z-stackup.
FR4, a common printed circuit board material, has been used in the past as an interconnect material with a folded over conducting tab, but this adds material thickness and increases Z-stackup. Metal stiffeners are a competitive edge, as they provide rigidity and planarity not available in standard material sets and offer a reduced package size in Z-stackup. But there has been no satisfactory solution for an electrically active ground connection to the back side of the stiffener.
In a cell phone, conductive materials which are floating (relative to the electrical potential of portions of the circuitry in the phone) can disturb the proper loading of an antenna, leading to poor radio connectivity. This can result in poor user satisfaction and potentially high return rates. Electrically unconnected metal in close proximity of noisy signals or components can couple and amplify noise within cell phone applications, affecting performance. In addition, corrosion susceptible metals or ferrous metals are not allowed in cell phone applications. Nickel-gold plated stainless steel can provide a thin, rigid, non-corroding and non-oxidizing stiffener, providing there is suitable means for attaching the stiffener to the flexible printed circuit board. Currently available conductive adhesives may provide low resistance as-assembled, but resistance increases as a result of reflow. Thus the industry has a need for a reliable, near-zero ohm connection of a flexible printed circuit board ground-to-metal-stiffener solution.